The oldest fossils reveal strange things about the Ediacaran animals that lived on Earth before the “Cambrian explosion”.

By analyzing the contents of the last meal eaten by the oldest animals known to have inhabited Earth more than 550 million years ago, scientists at the Australian National University have discovered new clues about the physiology of our earliest animal ancestors, known as the Ediacaran Animals or Ediacaran biota.

The Ediacaran Animals are thought to be the world’s earliest large animals, dating back 575 million years.

ANU scientists have discovered that animals eat bacteria and algae from the ocean floor. The findings, published in the journal Current Biology, provide more information about these strange organisms, including how they take in and digest food.

Scientists have analyzed ancient fossils that contain phytosterol molecules, natural chemical compounds derived from plants left over from animals’ last meal. By analyzing the molecular remains of the creature’s diet, scientists were able to prove that the slug-like organism Kimberella had a mouth and gut and digested food similar to modern animals. Researchers say it is probably one of the most advanced Ediacaran creatures.

The ANU team also discovered another species that could reach 1.4 meters in length and had a rib-like design on its body, but was simpler and lacked a mouth, digestive system or eyes. Instead, it was eaten by a strange animal called Dickinsonia as it moved along the ocean floor.

“Our findings show that the animals of the Ediacaran biota that lived on Earth before the ‘Cambrian explosion’ of modern animal life were a mixed bag of outright oddities like Dickinsonia,” lead author Dr. Ilya Bobrovsky says. and more advanced animals, such as Kimberella, have some of the same physiological characteristics as humans and other modern animals.

Kimberella and Dickinsonia are part of the Ediacaran biota family, which lived on Earth about 20 million years before the Cambrian explosion, a major event that changed the course of evolution for all life on Earth. Their structure and symmetry are unlike anything else available today.

“The Ediacaran biota are indeed the oldest fossils large enough to be seen with the naked eye, and they are the origin of us and all animals that exist today. These creatures are our deepest visible roots,” adds the author.

Algae are a valuable source of food and energy and may have helped Kimberella develop, according to study co-author Professor Jochen Brocks.

“Energy-rich food may explain why the organisms of the Ediacaran biota were so large. “Almost all fossils that preceded the Ediacaran biota were single-celled and microscopic in size,” adds Professor Brocks.

Using advanced chemical analysis techniques, ANU scientists were able to extract and study the sterol molecules in the fossil tissue. Animals contain cholesterol, so the ANU team confirmed in 2018 that the Ediacaran biota was among our earliest known ancestors.

The unique properties of the chemicals revealed the diet of the animals shortly before they died. Professor Brox said it was difficult to tell the difference between the signatures of the creatures’ fat molecules, the remains of algae and bacteria in their guts, and molecules of decaying algae from the ocean floor trapped together in fossils.

“Scientists already knew that Kimberella scraped the algae that covered the sea floor and left feeding marks that indicated the animal’s gut. But it was only after analyzing the molecules in Kimberella’s gut that we were able to determine exactly what she was eating and how she was digesting it,” adds Professor Brox.

“Kimberella knew exactly which sterols were good for it and had a developed small intestine to filter out the rest.

“It was a Eureka moment for us; Using chemicals preserved in fossils, we can now make the intestines of animals visible, even if their intestines have been decaying for a long time. We then applied this technique to strange fossils like Dickinsonia to understand how it fed, and discovered that Dickinsonia does not have a gut.

Fossils of both Kimberella and Dickinsonia were discovered in 2018 by Dr. Bobrovsky on rocky outcrops near the White Sea in Russia, in a wild region full of bears and mosquitoes.

Source: 10.1016/j.cub.2022.10.051